Should/Could I start over after ICH?


Reefing newb
I'm barely a novice, so gentle please!

My kid got a 5 gal tank. We followed all the proper procedures to get set up and put in 3 neon tetras (water tests were in the "good" zone). They did fine for a few weeks. We tested the water again and it was good so we decided to add two Kohaku Swordtails.

All went fine the first two weeks and then within a 2 hr time a tetra died. There were no symptoms. One minute he was swimming around fine, and two hours later when my kid looked it was on the bottom of the tank dead. Two days later a swordtail passed. Still no symptoms, or warning sign. We took water in to be tested and it was "ok". The next day the swordtail started developing white spots. I compared pictures and it looked exactly like ICH. We started the recommended treatment from the store but within 24 hours every other fish was dead.

It seems the recommended fallow time is 76 days.

Could I drain all the water, sterilize the tank and start over? Or should I let it sit empty? Are there other options?